[cups-devel] CUPS_PRINTER_REMOTE for automatically added printers on Linux

Hi,

I've trying to debug an issue for a while in our Linux-based operating
system [1] that has been an annoyance for our users in certain
scenarios, and I thought I'd ask here for some advice, because I'm
clearly missing things in the way, as I'm not sure whether it's a bug
in CUPS, or wrong usage of the CUPS API from gnome-settings-daemon.

FWIW, I'm using CUPS 2.2.4, which I believe is the latest release, and
the problem observed could be summarized as this:

When using our OS in busy network environments like coffee shops and
coworking spaces a notification often pops up to let you know that a
shared printer (or many) has been automatically found and installed,
from among those shared by nearby Apple or Microsoft users. By poking
at the code involved (CUPS, which provides all the information related
to the printers, and gnome-settings-daemon (g-s-d, from this point
on), which is in charge of notifying these kind of events), it seems
clear to me that the intention in GNOME is NOT to report these events
for *remote* printers, as you can see in [2] and [3].

However, g-s-d *does* report those events in our OS, so I debugged it
and found that the problem is that the `printer-type` attribute for
those printers, as retrieved via cupsGetOption, does not contain
neither the CUPS_PRINTER_REMOTE nor the CUPS_PRINTER_IMPLICIT bits,
which is what g-s-d checks for (see [2]), therefore determining that
the printer is local, and thus reporting the notification to the user.

Doing some more debugging, I found that the `printer-type` attribute
reported by CUPS is not empty at all, it actually makes kind of sense:
for instance, for the first printer found it contained the bits
CUPS_PRINTER_BW, CUPS_PRINTER_COLOR, CUPS_PRINTER_SMALL,
CUPS_PRINTER_MEDIUM, CUPS_PRINTER_VARIABLE... and
CUPS_PRINTER_NOT_SHARED. So, it's not that CUPS is not returning valid
data, it's definitely returning it.

What strikes me a bit, with my limited knowledge on the matter, is
that at some point I could see in the debug-verbose logs how CUPS was
setting the bits I would expect for the "printer-type" attribute right
when adding the printer, that is, the *very same bits I mentioned
above* (_BW, _COLOR, _SMALL...) plus CUPS_PRINTER_REMOTE, but no trace
of CUPS_PRINTER_NOT_SHARED.

With this observation in mind, I did poke the CUPS code to try to
understand why CUPS was apparently "replacing" _REMOTE with
_NOT_SHARED, but all I could find is the place where _NOT_SHARED is
added to the printer's type in [4], at which point it looks like the
_REMOTE bit is already gone, and I failed to see where that bit got
actually removed (if it was ever added).

So, after all this struggling I think I'm now at a point where I could
use some help :-). I guess my questions would be something like:

  * What flags for printer-type should I expect from an automatically
added printer in Linux? _REMOTE?, _IMPLICIT? _DISCOVERED? Other?
  * Is it correct to determine whether a printer is remote by doing
something like what g-s-d does in [2] and [3]? Should it be done in a
different way?
  * Is there perhaps a configuration option in CUPS that affects this behaviour?
  * Any other comment/advice that you might find interesting to help
me keep debugging this issue?

For the sake of completeness, this is what `lpinfo -v` prints when I
run it from the terminal after those printers got automatically added
by my CUPS instance, just by connecting to the local network:

```
network dnssd://Canon%20MG4100%20series%20%40%20moucho._ipp._tcp.local/cups?uuid=5103d1bd-3bfd-3be9-598e-825bbcf3f291
network dnssd://EPSON%20XP-412%20413%20415%20Series%20%40%20moucho._ipp._tcp.local/cups?uuid=fd210d3f-28bb-3e6c-5d43-f3dfe564aeb5
network dnssd://HP%20LaserJet%20Professional%20P1606dn%20%40%20MacBook%20Pro._ipp._tcp.local/cups?uuid=a763098c-4512-3f50-5638-612450c829d1
network dnssd://RICOH%20Aficio%20MP%20C2500%20%40%20moucho._ipp._tcp.local/cups?uuid=561c2092-e732-3294-6dce-38018345d2fe
network dnssd://SHARP%20MX-3060N._printer._tcp.local/
network dnssd://Samsung%20ML-1610%20%40%20MacBook%20Pro._ipp._tcp.local/cups?uuid=1caadc2a-4e7d-3f74-5e1d-55af53304a99
```

Any help very much appreciated. Also, please feel free to ask me to
perform any more tests that might be interesting, if it's a bug in
CUPS I'd be very happy to spend time helping to fix it.

Thanks in advance and sorry for the long email.
Mario

[1] https://endlessos.com
[2] https://git.gnome.org/browse/gnome-settings-daemon/tree/plugins/print-notifications/gsd-print-notifications-manager.c#n505
[3] https://git.gnome.org/browse/gnome-settings-daemon/tree/plugins/print-notifications/gsd-print-notifications-manager.c#n164
[4] https://github.com/apple/cups/blob/d2123aee5550f3b984e5bd8b11a6abbbd9913485/scheduler/ipp.c#L5055